Method of purifying alkali metal hydroxides



umrso STATE destroy chlorates.

' sulfite material. I pound used may be employed in an amountequivalentto the stoichiometric amount necessary to Patented July 31,1951 METHOD OF PURIFYING; ALKALI. METAL v I .HYDROXIDES ChesterCiBrumbaugh, Painesville, Ohio, ass'ignor to Diamond Alkali Company,.Gleveland, hi0,,

a corporation of Delaware NoDrawing. Application September 17, 1-948,

Serial No.4'9',858

.1- This invention relates to chemical manufacture and more particularlyrelates'to a novel and eiiicient method of preparing alkali metalhydroxides of high purity, particularly with respect to lrraterials ofoxidizing. character, such asalkali -.i netal chlorates' This inventionis especially adapted to the production of pure caustic soda.

Large volumes of caustic soda which must meet stringentspecifications,especially as to content electrolytic caustic soda fromchlorates. Consequently, it has long been the desire of electrolyticvcaustic manufacturers to acquire a suitable method of chlorate removaland many attempts have been made without success to devise such amethod. The prior art methods have either been prohibitively expensiveor have not resulted in a suitablechlorate-free caustic product forrayon;

uses. Numerous of these prior efforts have been directed to the use ofvarious forms of iron to Such methods are unsuccessful, however, in thatthere is either toomuch chlorate or too much iron in the final product.The other methods of chlorate removal which have been tried include theextraction of chlorate from caustic soda, extraction of caustic sodafrom chlorate, inhibition of the oxidative'effect of chlorate, and thelike, all of which were too expensive for commercial use or wereinsufficiently effective.

. giTh'epresent. invention contemplates-the substantially completeremoval of chlorates from alkali meta-l. hydroxides, more particularlyfrom caustic soda solutions or from anhydrous caustic soda, either as anindependent step in the manufacture of caustic soda or preferablyandmore conveniently inthe course of obtaining caustic soda in highconcentration aqueous solution or in' anhydrous condition. -In'g'enera1,the objects oi lthe invention-areaccomplished by treating "caustic sodasolutions containing an undesirable quantity of chlorate ions withalkali metal sulfites, the alkali metal ion being suitably that of thealkali metalhydroxide being treated with In general, the sulfitecomdestroy the known amount of chlorate ion present in the material, theamount-of chlorate ion having'been previously-ascertained by analytical"methods. It has been found desirable to use somewhat of an excess ofthe stoichiometric amount, such as a 20% excess, but those skilled inthe art will appreciate that once the amount 6 Claims. (01. 23.--184') 2of" chlorate present. in the alkali-metal hydroxide, such as causticsoda, being treated has'bee'n determined, the specific amount of sulfitematerial in excess. of the stoichiometric amount necessary to reduce thechlorate will become somewhat a matter. of. choice.

Asnoted above, it is contemplated that the present invention maybepracticed with alkali.

metal hydroxide solutions and especially caustic soda solutions of moreor less concentration, it being, preferred to treat solutionsofelectrolytic caustic soda of 50% or greater concentration andparticularly sinoelthose solutions have especially high concentrations.of chlorate-relative to chemically obtained'caustic soda, the presentinvention has its particular, applicability in the removal of chloratefrom electrolyticcaustic soda, which is obtained by electrolysis ofsodium chloride solutions. I Y

The alkali metal hydroxides, more particularly caustic soda, maysuitably be treated with the sulfite compound, in the case ofcausticsoda, more suitably sodium sulfite, preferably at somewhatelevated temperatures, such as temperatures from C. to themeltingtemperature "of anhydrous caustic: soda, it being preferre'dingeneral to incorporate the. sodium sulfite as, an incident ofconcentration procedure, whereby higher temperatures willv be realized,Moreover, the sodium sulfite may, if desired, be added'asa solid saltor. may be added as a saturated aqueous solution or in dilute. aqueoussolution, depending upon the point of the. sodium hydroxide manufacturein which the addition is made.

The. present invention has the particular advantage. that it reduces,the sodium chlorate to a tolerable limit without adding any otherdifiiculties, or in the case of the use of sodium sulfite to causticsoda, any foreign ions to the caustic'soda, particularly foreignmetallic ions which are espeoially undesirable in this connection. Aminor increase in the concentration of sulfate ion in the ultimatealkali metal will, of course, be experienced in the practice of thisinvention. However, higher concentrations'of sulfate can much morereadily be tolerated, and what amounts in efiect to. an exchange ofchlorate for. sulfate, is therefore a desirable. improvement in thepurity of the alkali metal hydroxide.

The invention is particularly applicable in the reduction of the causticsoda to relatively anhydrous condition by heat exchange evaporatorsystems 30f Water removal and more particularly by" for uses-forwhichheretofore electrolytic caustic particularly was not available.

As pointed out above, it is in general preferred to employ an amount ofsodiumsulfite equal to a slight excess of the stoiochiometric amount ofsodium chlorate found in the caustic soda product. Thus, a typicalamount of sodium sulfite to be used per ton of anhydrous caustic sodaobtained will be based on sodium chlorate content, which is typical inthe particular electrolytic operation referred to. An example of suchoperation is a caustic soda solution obtained elec trolyticallycontaining 0.166% of sodium chlorate, for which purpose it has beenfound useful to employ 14.1 pounds of sodium sulfite'per ton ofanhydrous caustic soda, whereby the chlorate content has been reduced toabout 0.002%, a tolerable amount of this material for practically allindustrial purposes.

The present invention is applicable to all of the alkali metalhydroxides, including lithium, potassium, sodium, rubidium, cesium, andthe like. However, it will be appreciated inview of the commercialimportance of the product that it has its greatest significance asapplied to sodium hydroxide or caustic soda. Moreover, other sulfitesthan sodium sulfite, such as lithium, potassium, rubidium, and cesium,may be employed, it being particularly desirable to employ the sulfitewhich has a common ion with the hydroxide being treated. In accordancetherewith, it will be appreciated that sodium sulfite will be preferredfor treating caustic soda.

In order to inform those skilled in the art of the manner in which thepresent invention may preferably be practiced but in no sense to betaken as limiting upon the scope of the invention described herein, thefollowing examples are set EXAMPLE I Electrolytic caustic soda solutionpreviously concentrated to a concentration of about 50% in caustic sodais passed through a falling film type evaporator, followed by aseparator in which the products of the evaporator are impinged upon thewall of the separator, the entire system be ing maintained at a walltemperature in excess of the melting temperature of anhydrous causticsoda and as is more particularly described in the above referred toapplication of R. E. Hulme. Sodium sulfite aqueous solution in a ratioof 14.1 pounds of sodium sulfite per ton of caustic product from theevaporator is added to the feed of the evaporation system. The contentof chlorate in the entering caustic soda solution based on anhydrouscaustic is 0.166%. The content of chlorate in the material issuing fromthe evaporator is of the order of 0.002%.

EXAMPLE. II

The procedure above was followed, varying the amount of sodium sulfiteadded to the feed with the following results:

Addition of sodium sulfite to electrolytic feed While there have beendescribed various embodiments of the invention, the methods describedare not intended to be understood as limiting the scope of the inventionas it is realized that changes therewithin are possible and it isfurther intended that each element recited in any of the followingclaims is to be understood as referring to all equivalent elements foraccomplishing substantially the same results in sub stantially the sameor equivalent manner, it being intended to cover the invention broadlyin what ever form its principle may be utilized.

What is claimed is:

1. The method of purifying an alkali metal hydroxide with respect toalkali metal chlorate, which comprises contacting a mixture of saidhydroxide and said chlorate with an alkali metal sulfite at an elevatedtemperature.

2. The method as claimed in claim 1 in which the mixture of alkali metalhydroxide and alkali metal chlorate is contacted with sodium sulfite atan elevated temperature.

3. The method as claimed in claim 1 in which the amount of sulfite addedto the alkali metal hydroxide solution is at least equal to thestoichiometric equivalent of chlorate present therein.

4. The method of removing sodium chlorate from caustic soda solutionswhich includes the step of contacting a caustic soda solution containingsuch chlorate with sodium sulfite at an elevated temperature.

5. The method of purifying sodium chlorate contaminated caustic sodawith respect to said sodium chlorate which includes the step ofcontacting caustic soda solution of more than 50% concentration with anamount of sodium sulfite at least equal to the stoichiometric equivalentof sodium chlorate present with such caustic soda.

6. The method of dehydrating sodium chlorate contaminated caustic sodawhile purifying said caustic soda with respect to said sodium chloratewhich includes the steps of heating such caustic soda until all of thewater associated therewith has been removed, and contacting said causticsoda during the anhydrizing process with sodium sulfite.

CHESTER C. BRUMBAUGH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,982,241 Aydelotte Nov. 27, 19342,142,670 Eichelberger Jan. 3, 1939 2,207,595 Muskat July 9, 19402,258,545 Davies Oct. 7, 1941 2,403,789 Cummins July 9, 1946 2,404,453Osborne July 23, 1946 2,415,798 Pye et al Feb. 11, 1947 OTHER REFERENCESMcPherson and Hendersons General Chemistry, pp. 514-515, 3rd ed., Ginnand Co.,' N. Y.

Handbook of Chemistry and Physics, 28th ed., pp. 460-461, 464-465, Chem.Rubber Pub. Co., Cleveland, Ohio.

Hackhs Chemical Dictionary, 2nd ed., p. 860, P. Blackistons Son and Co.,Inc., Philadelphia.

1. THE METHOD OF PURIFYING AN ALKALI METAL HYDROXIDE WITH RESPECT TOALKALI METAL CHLORATE, WHICH COMPRISES CONTACTING A MIXTURE OF SAIDHYDROXIDE AND SAID CHLORATE WITH AN ALKALI METAL SULFITE AT AN ELEVATEDTEMPERATURE.